1. Field of the Invention
The present invention relates to generally to photolithographic masks and specifically to photolithographic masks having assist features and to methods for use thereof.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including, but not limited to, ultraviolet radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm) and extreme ultraviolet radiation (EUV); in principle, these terms also encompass X-rays, electrons and ions. Also herein, the invention is described using a reference system of orthogonal X, Y and Z directions, and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term “vertical” (Z) used herein is intended to refer to the direction normal to the substrate or mask surface or parallel to the optical axis of an optical system, rather than implying any particular orientation of the apparatus. Similarly, the term “horizontal” refers to a direction parallel to the substrate or mask surface or perpendicular to the optical axis, and thus normal to the “vertical” direction.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion or exposure area (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus C which is commonly referred to as a step-and-scan apparatus C each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
In general, lithographic apparatus contain a single mask table and a single substrate table. However, machines are becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and loading positions.
It is known to provide so-called “assist features” in masks to improve the image projected onto the resist and ultimately the developed device. Assist features are features that are not intended to appear in the pattern developed in the resist but are provided in the mask to take advantage of diffraction effects so that the developed image more closely resembles the desired circuit pattern. Assist features are generally “sub-resolution” meaning that they are smaller in at least one dimension than the smallest feature in the mask that will actually be resolved on the wafer. Assist features may have dimensions defined as fractions of the “critical dimension”, which is the smallest width of a feature or smallest separation between features in the mask and is often the resolution limit of the lithographic projection apparatus with which the mask is to be used. Note though that because the mask pattern is generally projected with a magnification of less than 1, e.g. ¼ or ⅕, the assist feature on the mask may have a physical dimension larger than the smallest feature on the wafer. Two types of assist features are known. Scattering bars are lines with a sub-resolution width placed on one or both sides of an isolated conductor to mimic proximity effects that occur in densely packed regions of a pattern. Serifs are additional areas of various shapes placed at the corners and ends of conductor lines, or the corners of rectangular features, to make the end of the line, or the corner, more nearly square or round, as desired (note in this context that assist features commonly referred to as “hammerheads” are regarded as being a form of serif). Further information on the use of scattering bars and serifs can be found in U.S. Pat. Nos. 5,242,770 and 5,707,765, for example, which are incorporated herein by reference.
Contact holes, or vias, in integrated circuits cause particular problems in imaging. Because the contact holes often have to be formed through numerous or relatively thick process layers previously formed on the wafer, they must be patterned into a relatively thick layer of photoresist, requiring an increased depth of focus in the aerial image of the mask pattern.